High pulse repetition rate gas discharge laser

ABSTRACT

A pulsed gas discharge laser operating at an output laser pulse repetition rate of greater than 4 kHz and a method of operating same is disclosed which may comprise a high voltage electrode having a longitudinal extent; a main insulator electrically insulating the high voltage electrode from a grounded gas discharge chamber; a preionizer longitudinally extending along at least a portion of the longitudinal extent of the high voltage electrode; a preionization shim integral with the electrode extending toward the preionizer. The preionizer may be formed integrally with the main insulator. The preionization shim may substantially cover the gap between the electrode and the preionizer. The apparatus and method may comprise an aerodynamic fairing attached to the high voltage electrode to present an aerodynamically smooth surface to the gas flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patent application Ser. No. 11/169,203, entitled HIGH PULSE REPETITION RATE GAS DISCHARGE LASER, filed on Jun. 27, 2005, which issued as U.S. Pat. No. 7,633,989, the contents of which are hereby incorporated by reference.

The present application is related to U.S. patent application Ser. No. 10/877,737, entitled HALOGEN GAS DISCHARGE LASER ELECTRODES, filed on Jun. 25, 2004, and of Ser. No. 10/815,387, entitled GAS DISCHARGE LASER CHAMBER IMPROVEMENTS, filed on Mar. 31, 2004, the disclosures of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is related to high pulse repetition rate, e.g., 4 kHz and above gas discharge laser systems, e.g., excimer and molecular fluorine laser or CO.sub.2 systems.

BACKGROUND OF THE INVENTION

Gas discharge lasers, e.g., KrF, ArF, F.sub.2, XeF, XeCl, CO.sub.2 and the like laser systems operating at pulse repetition rates of up to 6 kHz and above present certain operational problems, related, in part, to disturbances to the gas flow circulating around the lasing medium containing chamber, and particularly in the vicinity of the high repetition rate gas discharges occurring between elongated gas discharge electrodes as such electrodes are well known in the art. Applicants propose certain improvements and modifications to existing assemblies comprising, e.g., electrodes, a preionizer(s), a main insulator(s) and electrode supports.

SUMMARY OF THE INVENTION

A pulsed gas discharge laser operating at an output laser pulse repetition rate of greater than 4 kHz and a method of operating same is disclosed which may comprise a high voltage electrode having a longitudinal extent; a main insulator electrically insulating the high voltage electrode from a grounded gas discharge chamber; a preionizer longitudinally extending along at least a portion of the longitudinal extent of the high voltage electrode; a preionization shim integral with the electrode extending toward the preionizer. The preionizer may be formed integrally with the main insulator. The preionization shim may substantially cover the gap between the electrode and the preionizer. The apparatus and method may further comprise a high voltage electrode; a main insulator insulating the high voltage electrode from a grounded gas discharge chamber; the high voltage electrode being disposed in an electrode receiving pocket in the main insulator and formed to present an elongated discharge receiving area facing another electrode within the gas discharge chamber, with adjacent side surfaces of the high voltage electrode slanting away from an outer extent of side walls of the pocket in the longitudinal extent of the pocket and the high voltage electrode, forming a gas flow disturbance pocket; an aerodynamic fairing attached to the high voltage electrode and substantially closing the gas flow disturbance pocket and presenting an aerodynamically smooth surface to the gas flow. The main insulator may be formed to present an aerodynamically smooth face in the direction of the gas flow and the fairing aerodynamically smooth surface being generally aligned with the main insulator aerodynamically smooth surface. The aerodynamic fairing may be on one side of the elongated discharge receiving area and a preionizer may be positioned on the other side of the elongated discharge receiving area. The one side is the downstream side or the upstream side. The aerodynamic fairing may comprise a pair of aerodynamic fairings one on each side of the elongated discharge receiving area. The apparatus and method may comprise an aerodynamic block presenting an extension of the aerodynamically smooth surface of the main insulator. The high voltage electrode may be formed with an integral shim extending toward the preionizer and presenting an aerodynamically smooth surface to the gas flow. The apparatus and method may further comprise an elongated electrode support bar formed to have an elongated dugout portion; an elongated electrode attached to the electrode support bar in the elongated dugout portion; an upstream fairing positioned in the elongated dugout portion presenting with the electrode support bar and electrode an aerodynamically smooth surface to a gas flow past the electrode. The apparatus and method may further comprise a downstream fairing positioned in the elongated dugout portion presenting with the electrode support bar and the electrode an aerodynamically smooth surface to a gas flow past the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view orthogonal to a longitudinal extension of electrodes in a gas discharge laser according to aspects of an embodiment of the present invention according to which ideal or optimized gas flow through the gas discharge region between the electrodes of such a gas discharge laser were tested;

FIG. 2 shows the cross-sectional view of the apparatus of FIG. 1 with gas discharge electrical components formed to closely approximate the ideal or optimized flow conditions according to aspects of an embodiment of the present invention;

FIGS. 3 and 4 show cross sectional views of a shimmed electrode according to aspects of an embodiment of the present invention;

FIG. 5 shows a perspective orthogonal view of a portion of a shimmed electrode according to aspects of an embodiment of the present invention;

FIGS. 6 and 7 show a side view of an electrode fairing according to aspects of an embodiment of the present invention;

FIG. 8 shows a cross-sectional view of the electrode fairing of FIGS. 6 and 7 at cross-sectional lines 8-8 in FIG. 6;

FIGS. 9-11 show cross-sectional views at different points along the elongated extent of the electrodes of a gas discharge laser in which the other of the electrodes and its mounting and fairings are formed according to aspects of an embodiment of the present invention for more optimized or idealized aerodynamic gas flow performance;

FIGS. 12 and 13 show aspects of embodiments of the present invention in which an electrode has a pair of fairings on either side of a discharge receiving portion of the electrode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to allow for higher repetition rate operation of e.g., gas discharge lasers, e.g., excimer lasers, such as ArF, KrF, XeF, and XeCl lasers, or F.sub.2 lasers or CO.sub.2 lasers or the like, e.g., at laser output light pulse repetition rates of up to and exceeding 6 kHz, applicants had expected that the blower 18 motor speed would have to increase about 50% above those needed for currently used lasers of such a type at around 4 kHz pulse repetition rates. This can create a need for tremendously higher blower 18 power, increasing on the order of as the cube of the speed increase. According to aspects of embodiments of the present invention applicants propose certain improvements to the flow path of the laser gas being moved by the blower 18 motor within the laser chamber in order to reach the required gas flow amounts (basically gas flow speed, although those skilled in the art will realize that the flow speed is not a constant either in the elongated longitudinal electrode axis direction or a direction perpendicular to that direction, due, e.g., to flow restriction of surrounding components, flow resistance at the boundaries of the flow region, etc.).

In the past such parameters as arc free blower speed (“AFBS”) have been measured to determine, e.g., the blower speed needed. The blower speed necessary is determined to be able to move sufficient gas away from the discharge region 40 between the elongated electrodes 10, 34, 36 in such devices. This is necessary to on the one hand assure the next sequential discharge is a good one (maintains the same laser output light energy for a given applied voltage through the discharge formed lasing medium) because the gas through which the discharge occurs to form the lasing medium is “fresh”. On the other hand the gas flow sufficiency is determined by being able to avoid arcing between one of the electrodes 10, 34, 36 and the grounded chamber walls or between the electrodes through ions present in the gas through which the prior discharge occurred between the electrodes. If such ions remain close enough to the electrodes 10, 34, 36 to form an electrical path to the chamber wall at the time when the high voltage one of the electrodes is raised in voltage to ultimately cause the discharge through the lasing medium laser gas such arcing can occur. The latter effect also, to among other things, results in instability of the laser output light pulse energy from pulse to pulse.

FIG. 1 shows a cross-sectional view of a test fixture that applicants used to test the gas flow properties of an “idealized” flow path for the gas through the discharge region 40 in the gap between electrodes 10, 34, 36, which are, along with surrounding elements simulated by a cathode blank 14 and an anode blank 16. The cathode blank 14 simulates an existing cathode 36 and adjacent portions of a main insulator 20 (shown in FIG. 2) without gaps 21A-C that have traditionally been present in such assemblies, e.g., as shown in FIG. 9 around the cathode 36. In addition flow smoothing blocks 44 were inserted to fill in open regions 44A on either side of the lateral extent of the main insulator 20, which open regions 44A are also shown in FIG. 9 in the vicinity of either side of the main insulator 20. The flow smoothing blocks 44 contain a plurality of narrow slits 38 that allow for the current returns to pass through the smoothing blocks 44.

Applicants found that this “idealized” gas flow path including an essentially uniform upper inlet gas flow surface 45 and upper exit gas flow surface 46 in combination with respective opposite lower inlet gas flow surface 47, which may correspond to an arcuate shaped surface, e.g., formed by an anode fairing 53 or a portion of an anode support bar 42 and an exit gas flow surface formed, e.g., by an downstream anode fairing 51, e.g., as shown in FIGS. 12 and 13 or by a portion of the anode support bar itself, as shown, e.g., in FIGS. 9-11 and discussed below.

Applicants found that measured gas flow speeds were indicative of the fact that the necessary blower speed with such an aerodynamically “idealized” gas flow path for, e.g., an increase in pulse repetition rate by 50% from 4 kHz to 6 kHz, was achievable and not the expected 50% increase in blower speed with the concomitant increase in required blower power supply. The idealized smooth upper 45, 47 and lower surfaces 47 remove, e.g., the effects of turbulence and flow separation created, e.g., by gaps 21A-C and open regions 44A between components of the laser system in the vicinity of the discharge region 40 and/or otherwise in the gas flow path from the blower 18 in a downstream direction 18A to the portion 12A of the lasing chamber 12 beyond the downstream extent of the anode support bar 42. The idealized smooth upper 45, 47 and lower surfaces 47 reduce the amount of turbulence, flow separation, eddies, and the like in the gas flow through the discharge region 40.

As can be seen from the illustrative partly schematic view in FIG. 2 according to aspects of an embodiment of the present invention, within the requirements for using separate parts, e.g., the cathode 10 and main insulator 20 having an electrode pocket 20A, the preionizer 25, current returns 60, 62, and the like, due, e.g., to their electrical, manufacturing, mechanical and structural properties and requirements, applicants propose according to various aspects of the present invention to provide a more aerodynamic gas flow path from the blower 18, in a downstream direction 18A, past upstream fairing 53 of the anode 34, through the gas discharge region 40 and past the anode 34 and its downstream faring 51 into the portion 12A of the chamber 12 beyond the anode support bar 42.

According to aspects of embodiments of the present invention this may include an integral cathode shim 32 and the cathode fairing 50. A main insulator 20 may have a through hole 22 for a ground rod for integral preionizer (“PI”) 30 operation. The integral shim 32 on the cathode 10 may be specifically shaped, e.g., not only to provide the pre-ionization function of bringing the high voltage on the cathode to the vicinity of the preionizer 25 ground rod 30, but also to shape/steer the gas flow efficiently toward the discharge region 40 between the electrode gap. In this manner, the traditional gaps 21A-C between the cathode 36, e.g., as shown in FIG. 9, and main insulator 20 and preionizer (where the preionizer 25 is an upstream preionizer 25, e.g., as shown in FIGS. 9 and 10, and providing a surface 100A, 102A exposed to the passing gas flow that is more like the aerodynamically smooth portion of the surface 45 in the “idealized” gas flow path of FIG. 1.

Since downstream flow separation on the anode assembly has been shown to increase the arc-free blower speed, according to aspects of an embodiment of the present invention a commensurate downstream/fastback fairing 51 (as shown in FIGS. 12 and 13 on the anode 42 may be matched with as a corresponding downstream/fastback fairing 50 for the cathode 10. The cathode fairing 50, as is the anode fairing 51, may be made of an insulator, e.g., of the same ceramic material as is the main insulator 20.

Previously, this might have been implemented by complex machining of the cathode and fairing and bolting them together to retain the cathode's original shape. According to aspects of an embodiment of the present invention the cathode fairing 50 may be, e.g., a ceramic part, in the shape of a wedge 50, e.g., as shown in FIGS. 6 and 7. This wedge 50 with two integral mounting pads 52, 54 outboard of the cathode 10 (in the elongated direction) may then be affixed to the main insulator 20 or to the upper chamber half 76, e.g., with ceramic screws or metal screws like the anode fairing 51. Alternatively, a rounded portion 78 of the cathode, as illustrated, e.g., in FIG. 5 with respect to the shimmed cathode 10, may be partially machined away and the mounting pads 52, 54 attached there with metal or ceramic screws.

In addition, the gas flow blocks and current return attachment plate fairings which were part of the testing fixture of FIG. 1 may also be included in the embodiment of FIG. 2 to further give the effect of the “idealized” gas flow passage in a downstream direction 18A across the upper inlet surface 45 and upper exit surface 46 of the gas flow path past the electrodes 10, 34 in order to make the surfaces more closely approach

the aerodynamically smooth surfaces of the “idealized” gas flow passage of FIG. 1.

As shown in FIGS. 12 and 13, the cathode fairings 100, 102 and 104, 106, respectively, may be used on both sides of the cathode 36, with the wedge shaped fairings 100, 102 similar to the one shown in FIG. 2, as shown in FIG. 12, and more rectangularly shaped fairings 104, 106 shown in FIG. 13. These fairings 104, 106, as illustrated, may, e.g., have noses 110 shaped to more conform to rounded edges of, e.g., the preionizer 25 on the upstream (e.g., fairing 106) or downstream (e.g., fairing 104) side, if any, or the main insulator 20 on either the upstream side or downstream side, or both, as applicable, and with flat protrusions 112 extending toward the discharge receiving portion of the cathode 36′.

According to aspects of an embodiment of the present invention the gas flow in the vicinity of the discharge region 40 may be further improved by approaching an idealized flow region while maintaining the necessary distinct nature of certain parts, e.g., because they are made of different materials or are not easily fabricated as a single piece, etc. The effects of turbulence and aerodynamic drag and eddy forming protrusions in the neighborhood of the discharge region 40, among other things which impact the gas flow properties, and, therefore, also the AFBS for a given configuration and repetition rate, can be significantly ameliorated on the side of the other electrode, as well as the side of the cathode as noted above.

In the past, the anode support bar in lasers sold by applicants' assignee, Cymer, Inc., e.g., models 6XXX and 7XXX laser systems, have had an anode support bar 42, 54 with a flat upper surface 42A, 54A, such as illustrated in FIGS. 1, 2, 12 and 13, to which have been attached an anode 34 and anode fairings 51, 53. The flat anode bottom surface 34C is in contact with the flat upper surface of the anode support bar 54. FIG. 13 also shows an anode shim 130A disposed between the flat anode bottom surface 34C and the flat upper surface of the anode support bar 54. The flow shaping anode fairings 51, 53 and anode 34 itself, extending above the anode support bar 42, 54, have been shown to also provide locations for drag, turbulence and flow separation, etc.

These can be substantially eliminated by, e.g., providing an anode support bar 42 such as illustrated in FIGS. 9, 10 and 11, wherein, e.g., the upper surfaces 72A and 42B of the anode support 42 and the upper surface 90A of upstream fairing 53 and the upper surface 92A of the downstream fairing 51 are not flat, but are shaped (e.g., curved or slanted as shown in FIGS. 9, 10 and 11) for flow aerodynamics, and the anode support bar 42 contains, a dugout region 80 having an upstream sidewall 80A, an upstream sidewall outer extent 80A′, a downstream sidewall 80B, a downstream sidewall outer extent 80B′ (as shown in more detail in FIG. 11) and a bottom surface 80C, into which may be inserted anode 34 and upstream fairing 53 and downstream fairing 51. The anode 34 includes an upstream side surface 34A slanting away from the upstream sidewall 80A of the dugout region 80 and forming an obtuse angle 80D relative to the bottom surface 80C of the dugout region 80, as shown in FIG. 11. The anode 34 also includes a downstream side surface 34B slanting away from the downstream sidewall 80B of the dugout region 80 and forming an obtuse angle 80E relative to the bottom surface 80C of the dugout region 80, as shown in FIG. 11. In this manner, the entire anode assembly can be made to present a more seamless aerodynamic flow profile to the gas flow through the discharge region 40. The front portion 72 of the anode support bar 42 forming a cutoff for the blower fan 18 may be made of separate piece as a manufacturing convenience to allow for the attachment of the upstream current returns attached to an attachment strip 70 that is secured between the front portion piece 72 and the anode support bar 42 with the front cutoff portion 72 attached to the anode support bar by screws 74. However, for purposes of the present application, according to aspects of embodiments of the present invention it may be considered part of the anode support bar 42. The upstream side surface 34A of the anode 34 can be separated from the upstream sidewall 80A of the dugout region 80 by an upstream gap 34A′. The downstream side surface 34A of the second high voltage electrode 34 can be separated from the downstream sidewall 80B of the dugout region 80 by a downstream gap 34B′, as shown in FIGS. 12 and 13.

The downstream current return tines 62 may be fastened to the anode support bar 42 by a strip 64 held in place by a clamp plate 66 attached to the anode support bar 42 by screws 68. The fairings 53, 51 may be connected to the anode support bar by screws 86, with the upstream fairing having a brass washer 82 and the downstream fairing having a crescent washer 84. The washers may be alternated from side to side along the length of the electrode fairings 53, 51.

To allow for higher repetition rate laser operation without the need for tremendously higher blower power, applicants also propose to replace a blade/dielectric electrode, e.g., an anode as originally designed and now implemented in certain laser products of the applicants assignee, Cymer, Inc., e.g., models 6X10 and 7000, with the anode extending in height above the adjacent fairings. This was done at least in part to ensure a properly attached discharge even after the anode had worn down after some use. But at higher rep rates up to and above 6 kHz applicants have discovered a flow penalty is paid for this protrusion into the gas flow. Applicants have also discovered that reefing electrodes that develop a natural reef from the reaction of lasing gas, e.g., fluorine with material in the electrode with exposure to the lasing medium in the lasing gas caused by high voltage discharge between the electrodes in the laser system or which are provided with an artificial reef there is very little or non-existent wear over electrode life. Therefore, according to aspects of an embodiment of the present invention applicants propose to, e.g., have the electrode, e.g., the anode height matched with the fairing height, or perhaps even slightly lower than the fairing height to allow for the reef to grow where the electrode and the type of laser system naturally grow reef coatings to allow the reef coating to extend essentially to the fairing 53, 51 height. For example, as illustrated in FIGS. 9-11 the anode can be “even with” the adjacent fairing(s) 53, 51 when it is, e.g., within about +−.0.5 .mu.m from the highest extension of the respective fairing(s) 53, 51, which may also be true for other embodiments with non-reefing electrodes being “even with” the fairing height(s).

It will be understood by those skilled in the art that the aspects of embodiments of the present invention disclosed above are intended to be preferred embodiments only and not to limit the disclosure of the present invention(s) in any way and particularly not to a specific preferred embodiment alone. Many changes and modification can be made to the disclosed aspects of embodiments of the disclosed invention(s) that will be understood and appreciated by those skilled in the art. The appended claims are intended in scope and meaning to cover not only the disclosed aspects of embodiments of the present invention(s) but also such equivalents and other modifications and changes that would be apparent to those skilled in the art. In addition to changes and modifications to the disclosed and claimed aspects of embodiments of the present invention(s) noted above the others could be implemented.

While the particular aspects of embodiment(s) of the HIGH PULSE

REPETITION RATE GAS DISCHARGE LASER described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. .sctn. 112 are fully capable of attaining any above-described purposes for, problems to be solved by or any other reasons for or objects of the aspects of an embodiment(s) above described, it is to be understood by those skilled in the art that the presently described aspects of the described embodiment(s) of the present invention are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present invention. The scope of the presently described and claimed aspects of embodiments fully encompasses other embodiments which may now be or may become obvious to those skilled in the art based on the teachings of the Specification. The scope of the present HIGH PULSE REPETITION RATE GAS DISCHARGE LASER is solely and completely limited by only the appended claims and nothing beyond the recitations of the appended claims. Reference to an element in such claims in the singular is not intended to mean nor shall it mean in interpreting such claim element “one and only one” unless explicitly so stated, but rather “one or more”. All structural and functional equivalents to any of the elements of the above-described aspects of an embodiment(s) that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Any term used in the specification and/or in the claims and expressly given a meaning in the Specification and/or claims in the present application shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as any aspect of an embodiment to address each and every problem sought to be solved by the aspects of embodiments disclosed in this application, for it to be encompassed by the present claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element in the appended claims is to be construed under the provisions of 35 U.S.C. .sctn. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited as a “step” instead of an “act”. 

We claim:
 1. A pulsed gas discharge laser operating at an output laser output light pulse repetition rate of greater than 4 kHz comprising: a cathode having a first elongated discharge receiving area; an anode support bar having a dugout region, the dugout region having a substantially flat bottom surface; an anode disposed in the dugout region of the anode support bar, the anode having: a second elongated discharge receiving area facing the first elongated discharge receiving area of the cathode, a discharge region defined between the first elongated discharge receiving area and the second elongated discharge receiving area; a substantially flat anode bottom surface disposed on the substantially flat bottom surface of the dugout region; an upstream side surface; a downstream side surface, the second elongated discharge receiving area having a first width between upstream side surface and the downstream side surface, the upstream side surface having a first linear slope forming a first obtuse angle to the substantially flat bottom surface of the dugout region, the downstream side surface having a second linear slope forming a second obtuse angle to the substantially flat bottom surface of the dugout region, the first obtuse angle being opposite from the second obtuse angle the flat anode bottom surface having a second width greater than the first width between upstream side surface and the downstream side surface; a preionizer adjacent to the cathode, the preionizer and the cathode being disposed in an electrode pocket, the electrode pocket and the cathode, forming a gas flow disturbance pocket; a first aerodynamic cathode fairing attached to the cathode and substantially closing the gas flow disturbance pocket and having an aerodynamically smooth surface to the gas flow in a downstream flow path defined as flowing from a blower and through the discharge region, and the first aerodynamic cathode fairing is disposed on a first side of the first elongated discharge receiving area and the preionizer being disposed on a second side of the first elongated discharge receiving area, the second side of the first elongated discharge receiving area being opposite from the first side of the first elongated discharge receiving area.
 2. The apparatus of claim 1, wherein the upstream side surface of the anode is separated from the upstream sidewall of the dugout region by an upstream gap and the downstream side surface is separated from the downstream sidewall of the dugout region by a downstream gap.
 3. The apparatus of claim 1, further comprising an upstream anode fairing disposed between the upstream side surface and the upstream sidewall of the dugout region and a downstream anode fairing disposed between the downstream side surface and the downstream sidewall of the dugout region.
 4. The apparatus of claim 3, wherein a portion of an upper surface of the anode support bar, an upper surface of upstream anode fairing and an upper surface of the downstream anode fairing are aligned to form a single anode aerodynamically smooth surface to the gas flow from the blower and through the discharge region.
 5. The apparatus of claim 1, wherein the electrode pocket is formed in a main insulator, the electrode pocket including: a first outer extent side wall; a second outer extent side wall; the cathode including: a first side surface adjacent to and extending from the first elongated discharge receiving area of the cathode toward the first outer extent side wall along a third linear slope the first side surface forms a first acute angle relative to the first outer extent side wall, defining a first gap; a second side surface adjacent to and extending from the first elongated discharge receiving area of the cathode toward the second outer extent side wall along a fourth linear slope the second side surface disposed opposite from the first side surface, the second side surface forms a second acute angle relative to the second outer extent side wall, defining a second gap; a second aerodynamic cathode fairing being attached to the first side surface of the cathode and substantially closing the first gap and presenting an aerodynamically smooth surface to the gas flow.
 6. The apparatus of claim 5, wherein the cathode is formed with an integral shim extending toward the preionizer and the integral shim forming a second aerodynamically smooth surface to the gas flow.
 7. The apparatus of claim 5, wherein the first aerodynamic cathode fairing includes: a wedge cross-sectional shape; and two integral mounting pads disposed outboard of the cathode in an longitudinal extent of the cathode, the two integral mounting pads being affixed to the main insulator. 